Hubble's Law: galaxies are travelling away from the Earth at speeds proportional to their distance, and the light from these galaxies experiences a shift towards the red end of the spectrum (Red Shift).

Theories

Hubble observed that nearly all galaxies had a redshift in their spectrum of frequencies, indicating that a type of Doppler Effect was occurring, due to their velocities away from the Earth. He expressed this initially as 'apparent velocity', since he was not sure enough to conclude that this was evidence of recession.

Equations

The Hubble Parameter (previously Hubble Constant) $H_0 ≈ (74.3 ± 2.1) {km}/{s⋅Mpc}$ expresses the relationship between the speed of recession of galaxies and their distance from Earth. This provides an estimate for the age of the universe from the Big Bang as 13.6 billion years.

Experiments/Discoveries

Red-Shift of galaxies, revealing that distant galaxies are receding from the Earth at speeds proportional to their distance.

Using the then world's largest telescope, the 2.5m Hooker Telescope, at the Mount Wilson Observatory, California, in 1923 he observed that the M31 Andromeda Nebula lay outside our Milky Way Galaxy, upturning previous and deeply embedded presumptions, and heralding a new age in astronomy.

Edwin Hubble, 1889 - 1953, American astronomer, whose work confirmed the General Relativity Theory model, and abolished the cosmological constant

Although Georges Lemaître (1894 - 1966) has been retrospectively established as the originator of the theory for the expanding universe with a precise origin, Hubble has been traditionally extolled by the popular media as the theory's inventor. Hubble's observations of Cepheid variable stars established that there were more than our one galaxy, and that other galaxies were travelling at great speeds away from us.

This discovery stunned the scientific community, and is one of the greatest leaps forward in our understanding of the universe. From this, Einstein's General Relativity field equations could be readjusted to their original form (with the 'fudge factor' of the cosmological constant, which Einstein had inserted to keep the universe static). Lemaître's brilliant and radical idea that the universe had had a moment of creation, rejected by the scientific community, was now vindicated, and became the established model.

Hubble's other work included the Hubble Sequence, a novel system for classifying galaxies, still the dominant system in use today.